Acceleration is obtained by differentiation of displacement two times. The actual acceleration is low even for a large displacement with a lower frequency. As examples, for a displacement of 10 micrometers at 160 Hz, the acceleration is 1 G; for a displacement of 10 m at 0.16 Hz, it is 1 G. In a vibration measurement at a lower frequency, the actual displacement is as small as 1 m maximum. For a low frequency of 1.6 Hz, for example, the acceleration is 0.1 G with displacement of 1 cm.
Accordingly, in order to measure a vibration of 0.1 to 10 Hz, it is necessary to measure acceleration as low as 0.1 to 0.01 G.
The inventors have filed an application for an acceleration sensor for use at lower frequencies as Japan Utility Model No. 63-103602. The mentioned acceleration sensor is constructed with a piezoelectric device enclosed by a conductive resin, an adiabatic member, and a thermally conductive member in that order. The enclosed piezoelectric device is mounted on an insulative substrate, and a capacitor, for preventing possible external inductive noises, is connected between the conductive resin and the electrically-conductive thermally conductive member. In this construction, the electrically-conductive resin can electromagnetically shield the piezoelectric device. The piezoelectric device is free from adverse effects of possible external electrical noises. With use of the adiabatic member and thermal conductive, also, it is possible to considerably reduce adverse effects due to a possible temperature change by external heating or cooling. The piezoelectric device 6, further, is electrically isolated from a matter to be measured by means of the insulative substrate. Thus, it cannot be subjected to possible inductive noises, such as an electrostatic induction and a potential to ground. The capacitor for preventing possible external inductive noises that is connected between the conductive resin and the electrically-conductive thermally conductive member can by-pass possible external radio-frequency noises. This is effective in higher precision measurement for low-frequency, low accelerations.
However, the prior art mentioned above must have the capacitor to prevent possible external inductive noises connected between the conductive resin and the electrically-conductive thermal conductive in order to by-pass possible external radio-frequency noises. This frequently proves to be an inconvenience in an environment, since the construction is complicated as such. In addition, the prior art must have an additional cable for connecting an output of the piezoelectric device to a separate signal processing electronic circuit. This also frequently proves to be an inconvenience in that the construction cannot be small-sized or handy and that possible external inductive noises could occur between the piezoelectric device and related electronic circuits. Further, since the piezoelectric member is not supported by a member having a linear expansion coefficient of less than the one, the sensitivity of the sensor cannot be constant with a change of environmental temperature.